CRETACEOUS LACUSTRINE ENVIRONMENTAL CHANGE OF THE SONGLIAO BASIN, NE CHINA: CASE STUDY OF SK-1 COREHOLE

The Songliao Basin is the largest Cretaceous oil and gas-producing lacustrine basin in China, with its greatest extent in the middle Cretaceous. A coring program (SK-1) has to date yielded 2485.89 m of continuous core ( 96.46%recovery) and provides significant material for Cretaceous research. The sequence of the core consists mainly of lacustrine sandstone, dark mudstone, shale and oilshale. It has been subdivided into the Quantou, Qingshankou, Yaojia, Nenjiang, Sifangtai and Mingshui formations in ascending order. Based on biostratigraphy, astronomical time, high-resolution magnetostratigraphic and SIMS U-Pb zircon analyses of the SK-1, the age can be constrained to an interval from Turonian to Maastrichtian.

Microfossils are at their best during the Late Cretaceous. A comprehensive study of property, assemblage character and sequence of the microfossils plays a positive role in environmental study. It shows that two large lake transgressions occurred during deposition of the Qingshannkou Formation Unit 1, Nenjiang Formation Units 1 and 2. Foraminifera are the first time observation in the Nenjiang Formation Units 1 and 2. Nevertheless, the obtained foraminiferal fossils at first glance to indicate that seawater invaded into the Songliao basin during the early Nenjiang time when it involved with the largest lake transgression. The present study also provides the first reported carbon isotopic results, which were determined from ostracode carapaces from the SK-1. The results show an incredibly textured and rich isotopic record, some of which can be related to global climate and other portions to basin evolution.

The preliminary study indicates that lake water salinity changes as freshwater–brackish water–freshwater cycle, with sea water incursion during its maximum transgression. The transgressions resulted in the development of anoxic environments in the deep parts of the basin. This implies that greenhouse anoxia might have occurred in the Songliao Lake. The elevated burial of organic carbon in the Songliao Basin at this time points to a further similarity with the world’s oceans.